Applied Physics Letters, 2007,
to be published
Room temperature green light emissions from nonpolar cubic
InGaN/GaN multi quantum wells
Shunfeng Li *, Jörg Schörmann, Donat. J. As, Klaus Lischka a)
Universität Paderborn, Department of Physics, D-33098 Paderborn, Germany.
Abstract
Cubic InGaN/GaN multi quantum wells (MQWs) with high structural and optical
quality are achieved by utilizing free-standing 3C-SiC (001) substrates and optimizing
InGaN quantum well growth. Superlattice peaks up to 5th order are clearly resolved in Xray diffraction. We observe bright green room temperature photoluminescence (PL) from
c-InxGa1-xN/GaN MQWs (x=0.16). The full-width at half maximum of the PL emission is
about 240 meV at 300 K. The PL intensity increases with well thickness, proofing that
polarization fields which can limit the performance of the wurtzite III-nitride based
devices are absent. The diffusion length of excess carriers is about 17 nm.
Electronic mail: lischka@upb.de
1
Group III-nitrides crystallize in the stable wurtzite (hexagonal) structure or in the
metastable zincblende (cubic) structure. Currently, state-of-art nitride-based devices are
grown along the polar c-axis of the wurtzite III-nitride unit cell. Due to strong
polarization field along the c-axis, the electron-hole pairs within quantum wells are
separated, which results in a red-shift of optical transitions and a reduced oscillator
strength. This so called quantum confined Stark effect (QCSE) [1,2] can reduce the
efficiency of optoelectronic devices which employ InGaN/GaN quantum wells in the
active region. Thus in highly efficient wurtzite InGaN/GaN multi quantum well (MQW)
based devices, the InGaN well has a thickness of less than 3 nm to avoid degradation of
the light emission by the QCSE [3]. Polarization field effects can also be detrimental for
electronic devices, e.g. normally off field effect transistors [4].
To overcome these problems wurtzite GaN grown along non-polar direction, e.g.
a- and m- directions have been explored by several groups [5-7]. Recent results reveal the
absence of polarization induced electric field in a-plane and m-plane grown wurtzite IIInitride MQWs. However, the structural and optical properties of nonpolar wurtzite IIInitrides are still inferior compared to their mature c-plane grown counterpart.
Due to the centro-symmetric unit cell of cubic III-nitrides polarization and
piezoelectric fields are absent when they are grown in an (001) direction. Furthermore,
due to the relatively narrower bandgap (about 200 meV) of cubic III-nitrides, InGaN
layers containing less In can be used to extend the light emission to the green spectral
range, which is still a challenge for high efficiency wurtzite III-nitride based
optoelectronic devices.
In this paper we show that by optimizing the growth conditions and employing
free-standing 3C-SiC (001) substrates, cubic InGaN/GaN MQWs with improved quality
can be fabricated. Their optical and structural properties are comparable to that of nonpolar (11-20) a-plane wurtzite InGaN/GaN MQWs. The absence of any polarization
fields in our c- InGaN/GaN MQWs is demonstrated. We report strong green emission
from c- InGaN MQWs with 7.8 nm thick In0.16Ga0.84N well layers.
Samples investigated in this work were grown on free-standing 3C-SiC (001)
substrates by molecular beam epitaxy (MBE). The MBE system is equipped with an
Oxford RF plasma source for activated nitrogen. Indium and Gallium is evaporated from
2
conventional Knudsen cells and the metal fluxes were adjusted by the source
temperatures. Prior to growth, the 3C-SiC substrates were chemically etched and
annealed for 10 hours at 500 °C. GaN buffer layers were grown at 720 °C with a growth
rate of 100 nm/h under controlled Ga-rich conditions. On top of the c-GaN buffer layers
6-period InxGa1-xN/GaN MQWs were deposited. The InGaN layers were grown at 610 °C
under slightly In-rich growth conditions. Indium rich growth helps to improve the
interface quality by utilizing the indium surfactant effect [8]. However, excess In flux
leads to an enhanced In surface segregation, which is detrimental for the interface quality.
Therefore, growth was interrupted after the deposition of each quantum well with the
InGaN layer at growth temperature. These interruptions have been proven to remove
effectively the surface In atoms and relieve the In segregation effect [9]. In a next step a
thin GaN layer was deposited to avoid the decomposition of the InGaN layer during the
following ramping-up of the substrate temperature. The remaining GaN barrier layers
were grown at a substrate temperature of 720 °C. The InGaN well thickness was varied
from 2 to 8 nm, the barrier thickness was about 12.0 nm unless otherwise mentioned.
Photoluminescence (PL) measurements were performed at room temperature and
2 K using excitation by a 325 nm HeCd laser. The light intensity on the samples was
about 10W/cm2. The luminescent light was dispersed by a monochromator and detected
by a cooled photomultiplier. A Philips X’pert high resolution x-ray diffractometer
(HRXRD) was used to characterize the structural properties of the samples. The HRXRD
diffraction profiles of our c-InGaN/GaN QWs were analyzed by a dynamic diffraction
model which allowed deriving the parameters of the quantum wells.
Figure 1 displays experimental and simulated HRXRD profiles of a typical 6period c-InxGa1-xN/GaN MQW. Pronounced superlattice satellite peaks up to the fifth
order are observed indicating a high structural quality of the GaN/InGaN interfaces. A
simulation using dynamic diffraction theory has been performed; the results are also
shown in Fig. 1. The calculated curve agrees well with the experimental data, yielding a
well thickness of 3.4 nm, an indium mole fraction of 0.16 and a barrier thickness of 14.9
nm, respectively. A reciprocal space map of the (-1-13) Bragg reflex revealed that all
InGaN QWs are pseudomorphically grown on the c-GaN buffer.
Room temperature and 2 K PL spectra of a 6-period c-InxGa1-xN/GaN (x=0.12)
3
MQW are shown in Fig. 2.a and 2.b. The well thickness of this sample is 3.7 nm. Strong
quantum well emission at 2.57 eV and 2.65 eV is observed at 300 K and 2 K, respectively.
The full-width at half maximum (FWHM) of the InGaN emission are 240 meV and 190
meV for 300 K and 2 K, which compare favorably to the values reported for nonpolar aplane wurtzite InGaN/GaN MQWs with similar emission energy [5]. The two additional
peaks at 3.13 eV and 3.26 eV in Fig. 2a originate from the donor-acceptor pair transition
and bound exciton trasition in the GaN barriers, respectively [10].
Quantum wells enhance the light emission by increasing oscillator strength and
collecting the carriers generated within an effective diffusion length (LEDL) of the barriers.
We roughly estimate LEDL by comparing the PL intensity of this MQW sample with that
of a single quantum well sample with 4.5 nm thick well and similar In mole fraction.
Room temperature PL spectra of both samples were recorded under identical conditions.
Since the GaN emission is almost zero in the PL spectrum of MQW sample (see Fig.
2(a)), we conclude that all the electron-hole pairs generated in the barrier layers of the
MQWs either diffuse into the InGaN well layers or recombine nonradiatively, revealing
that LEDL exceeds the barrier thickness of the MQWs.
Assuming (i) the same carrier generation rates and nonradiative recombination
rate in the GaN barriers and the InGaN quantum wells, and (ii) no variation of the
excitation intensity in that region, where those electron-hole pairs are excited which
contribute to the emission, and (iii) surface recombination can be neglected we can
calculate the effective carrier diffusion length LEDL by the following equation:
2.6 ∗ ( LSW + 2 LEDL ) = 6 ∗ ( LMW + LB ) + LEDL
(1)
where LSW is the well thickness of the SQW, LMW and LB are the well and the barrier
thickness of MQWs, respectively. The factor of 2.6 on the left hand side is the ratio of the
integrated PL intensity from MQWs (Fig. 2(a)) and a SQW (not shown). The barrier
thickness of the MQWs sample is 10.3 nm. The thickness of the top layer of the MQW
sample is equal to the barrier thickness, therefore six LB appear on the right hand side of
Equ. (1). Using Equ. (1) we obtain an effective diffusion length of excess carriers in our
samples of about 17 nm. In our calculation we have neglected the coupling between QWs
since numerical calculations using a program of I.H.Tan et al [11] show that it is
extremely week for barriers as thick as in our InGaN/GaN MQW samples. We therefore
4
assume an equal recombination rate in the single QW and the individual quantum wells
of the MQW structure, respectively.
It has been shown that in h-InGaN/GaN quantum wells grown on c-plane
substrates the PL intensity decreases with increasing well thickness due to the separation
of electrons and holes by polarization fields [12]. In order to demonstrate the absence of
polarization field in our samples, we grew a series of c-InGaN/GaN (x=0.160±0.003)
MQWs with identical structural parameters except the quantum well thickness. Room
temperature PL spectra of these samples are depicted in Fig. 3. The respective well
thickness is indicated in the graphs. The c-InGaN quantum well emissions dominants all
spectra. With increasing well thickness a clear red-shift is observed. No emission from
the c-GaN barriers is visible, which manifests the high collection efficiency of the InGaN
QWs.
Figure 4 depicts the integrated PL intensity versus quantum well thickness. The
PL intensity increases monotonically with well thickness up to about 8 nm. This is in
clear contrast to what was observed with c-plane h-InGaN/GaN MQWs where the
emission intensity decreases with quantum well thickness [12]. This shows
experimentally that the polarization fields are absent in our c-InGaN/GaN MQWs.
The bandgap energy of cubic III-nitrides is about 200 meV smaller than that of
their hexagonal counterparts. Thus, for green or even longer wavelength light emission,
about 10% less In is necessary to be incorporated in c-InGaN QWs than it would be with
hexagonal III-nitride based devices. Furthermore, due to the absence of polarization field
in cubic InGaN quantum wells, the quantum well thickness can be tuned in a wider range
without degradation of the recombination efficiency. We are able to obtain strong 520 nm
green PL emission from c-InxGa1-xN/GaN (x=0.160) MQWs with 7.8 nm thick wells (see
Fig. 3). The width of the emission band is comparable to that of h-InGaN/GaN MQW
grown on non-polar planes emitting in the same wavelength range [13].
In summary, we have shown that by optimizing the c-InGaN growth conditions,
high quality c-InGaN/GaN MQWs have been grown on 3C-SiC substrates, which have
abrupt interfaces, as was manifested by the order (up to 5th) of superlattice peaks in Xray diffraction profile. The room temperature PL emission from 6-period cIn0.16Ga0.84N/GaN MQWs has a FWHM of about 240 meV and a peak wavelength of 520
5
nm in the green spectral region. The effective diffusion length of excess carriers in these
QWs is about 17 nm. The measured integrated PL intensity of c-InGaN/GaN MQWs
increases monotonically with the well thickness from 2 nm to about 8 nm which proves
that polarization fields do not exist in cubic InGaN/GaN quantum wells.
Acknowledgements
We would like to thank H. Nagasawa and M. Abe from SiC Development Center, HOYA
Corporation, for supplying the 3C-SiC substrates.
6
References
* present adress: Universität Karlsruhe, Institut für Angewandte Physik, 76128
Karlsruhe, Germany
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7
Figure captions
Figure 1: Experimental and simulated double-crystal X-ray diffraction profiles (omega-2
theta scans) of the symmetric (002) Bragg-reflection of a 6-period c-InxGa1-xN/GaN
MQW (x = 0.16) on 3C-SiC (001).
Figure 2: Photoluminescence spectra of 6-period c-InxGa1-xN/GaN MQWs (x=0.12)
measured at different temperatures. (a) 300 K. (b) 2 K.
Figure 3: Room temperature photoluminescence spectra of 6-period c-In0.16Ga0.84N/GaN
MQWs with different well thickness. The well thickness is indicated in the figure.
Figure 4: Integrated room temperature photoluminescence intensity of 6-period cIn0.16Ga0.84N/GaN MQWs versus well thickness. The curve is a guide for the eye.
8
intensity (a.u.)
10
5
10
4
10
3
c-In
0.16
Ga
0.84
c-GaN (002)
N/GaN
6-period MQW
SL 0
SL+1
102
SL-1
SL-2
10
1
10
0
SL-3
SL-4
SL-5
-1
10
36
exp.
sim.
37
38
39
2 theta (deg)
40
41
5x105
8000
c-In
Ga
0.12
5
4x10
N/GaN
T=300K
0.88
T=2K
6-period MQW
c-In0.12Ga0.88N/GaN
6-period MQW
3x105
FWHM=240meV
5
2x10
FWHM=190meV
4000
2000
5
1x10
0
1.5
PL intensity (a.u.)
PL intensity (a.u.)
6000
2
2.5
energy (eV)
3
3.5
0
2
2.5
3
energy (eV)
3.5
T=300K
c-In
Ga
0.16
N/GaN
0.84
PL intensity (a.u.)
6 -period MQWs
243meV
7.8nm
5.6nm
3.7nm
2.0nm
1.5
2
2.5
energy (eV)
3
3.5
6x10
5
integrated PL intensity (a.u.)
c-In
4x10
Ga
0.16
5x105
N/GaN
0.84
T=300K
6-period MQWs
5
3x105
2x10
5
1x10
5
0
0
2
4
6
well thickness (nm)
8